Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in: The concerted motion of two or more bound electrons governs atomic 1 and molecular 2,3 non-equilibrium processes and chemical reactions. It is thus a long-standing scientific dream to measure and control the dynamics of two bound and correlated electrons in the quantum regime. At least two active electrons and a nucleus are required to address such quantum three-body problem 4 for which analytical solutions do not exist, a condition that is met in the helium atom. While attosecond dynamics were previously observed for singleactive electron/hole cases 5-7 , such time-resolved observation of two-electron motion thus far remained an unaccomplished challenge. Here, we measure a 1.2-femtosecond quantum beat among low-lying doubly-excited states in helium and use it to reconstruct a correlated two-electron wave packet. Our experimental method combines attosecond transientabsorption spectroscopy 5,7-9 at unprecedented high spectral resolution (20 meV s.d. near 60 eV) with an intensity-tuneable visible laser field to couple 10-12 the quantum states from the weak-field to the strong-coupling regime. Employing the Fano resonance as a phasesensitive quantum interferometer 13 , we demonstrate the coherent control of two correlated electrons, which form the basis of most covalent molecular bonds in nature. As we show, such multi-dimensional spectroscopy experiments provide benchmark data for testing fundamental few-body quantum-dynamics theory. They also light a route for site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental reactions in chemistry and biology.Electrons are bound to atoms and molecules by the Coulomb force of the nuclei. Moving between atoms, they form the basis of the molecular bond. The same Coulomb force, however, acts repulsively between the electrons. This electron-electron interaction represents a major challenge in the understanding and modelling of atomic and molecular states, their structure and in particular their dynamics 2,3,14 . Here, we focus on the 1 P sp 2,n+ series 15 of doubly-excited states in helium below the N = 2 ionization threshold. They are excited by a single-photon transition from the 1 S 1s 2 ground state by the promotion of both electrons to at least principal quantum number n = 2. The states autoionize due to electron-electron interaction and their spectroscopic signature manifests as asymmetric non-Lorentzian line shapes. The latter were first observed in the 1930s 16 and attributed 17 , by Ugo Fano, to the quantum interference of bound states with the continuum to which they are coupled (Fig. 1c,d). The coupling is described by the configuration interaction V CI with the single-ionization continuum |1s,p, where one electron is in the 1s ground state and the other one is in the continuum with kinetic energy . The magnitude of V CI determines the lifetimes of the transiently bound states. In our case,...
Coffin-Lowry syndrome (CLS) is characterized by cognitive impairment, characteristic facial and digital findings and skeletal anomalies. The gene implicated in CLS encodes RSK2, a serine/threonine kinase acting in the Ras/MAPK signalling pathway. In humans, RSK2 belongs to a family of four highly homologous proteins (RSK1-RSK4), encoded by distinct genes. RSK2 mutations in CLS patients are extremely heterogeneous. No consistent relationship between specific mutations and the severity of the disease or the expression of uncommon features has been established. Together, the data suggest an influence of environmental and/or other genetic components on the presentation of the disease. Obvious modifying genes include those encoding other RSK family members. In this study we have determined the expression of RSK1, 2 and 3 genes in various human tissues, during mouse embryogenesis and in mouse brain. The three RSK mRNAs were expressed in all human tissues and brain regions tested, supporting functional redundancy. However, tissue specific variations in levels suggest that they may also serve specific roles. The mouse Rsk3 gene was prominently expressed in the developing neural and sensory tissues, whereas Rsk1 gene expression was the strongest in various other tissues with high proliferative activity, suggesting distinct roles during development. In adult mouse brain, the highest levels of Rsk2 expression were observed in regions with high synaptic activity, including the neocortex, the hippocampus and Purkinje cells. These structures are essential components in cognitive function and learning. Based on the expression levels, our results suggest that in these areas, the Rsk1 and Rsk3 genes may not be able to fully compensate for a lack of Rsk2 function.
Fano line shapes observed in absorption spectra encode information on the amplitude and phase of the optical dipole response. A change in the Fano line shape, e.g., by interaction with short-pulsed laser fields, allows us to extract dynamical modifications of the amplitude and phase of the coupled excited quantum states. We introduce and apply this physical mechanism to near-resonantly coupled doubly excited states in helium. This general approach provides a physical understanding of the laser-induced spectral shift of absorption-line maxima on a sub-laser-cycle time scale as they are ubiquitously observed in attosecond transient-absorption measurements.
We demonstrate a two-dimensional time-domain spectroscopy method to extract amplitude and phase modifications of excited atomic states caused by the interaction with ultrashort laser pulses. The technique is based on Fourier analysis of the absorption spectrum of perturbed polarization decay. An analytical description of the method reveals how amplitude and phase information can be directly obtained from measurements. We apply the method experimentally to the helium atom, which is excited by attosecond-pulsed extreme ultraviolet light, to characterize laser-induced couplings of doubly-excited states.
We report on the experimental observation of strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a freeelectron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of µJ. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, reveals a direct link between strongfield-induced energy and phase shifts of the doubly excited state and the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of few meV, induced by strong XUV fields. These results open up a new way of performing non-perturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.Quantum mechanics provides a consistent description of the structure and dynamics of atoms, the constituents of our macroscopic world. In particular, it describes how bound excited states in atoms are formed through the Coulomb interaction of the positively charged nucleus and the negatively charged electrons. With the obvious exception of the ground state, such states possess a finite lifetime, with singly excited states decaying through photon emission via the interaction with the radiation field. For two-electron excitations of neutral atoms, the Coulomb interaction between the electrons is much more effective such that at least one electron will eventually be ionized, which typically marks the leading contribution to the decay of the excited state for the case of light atoms. Thus ionization is a fundamentally important and very basic effect that accompanies the physics of multi-electron excitations in atoms [1]. An interesting situation arises if the interaction of such states with the radiation field is significantly increased which nowadays can be achieved by using extreme ultraviolet (XUV) or x-ray light sources. In addition, the properties of these radiation fields can often be well controlled, thus providing a unique toolbox for exploring the dynamics of excited states, e.g., by performing time-resolved investigations with lab-based attosecond high-order harmonic generation (HHG) sources [2,3], or facility-based femtosecond XUV/x-ray freeelectron lasers (FELs) [4,5]. The latter deliver particularly high intensities for XUV/x-ray nonlinear optics [6] with ultrafast time resolution and site-specific core-level access [7], and nowadays even approach the attosecond regime [8].The helium atom consists of two electrons bound to a nucleus, representing the ideal case of a Coulombic three-body system, which serves as a benchmark for developing a theoretical description [1,9,10] and most importantly also for controlling the dynamics of two bound electrons with stron...
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